The present invention relates generally to liquid ink recording apparatus or ink jet printers, and more particularly relates to such a recording apparatus including an efficient sheet or substrate heating and supporting assembly.
Liquid ink printers of the type frequently referred to either as continuous stream or as drop-on-demand, such as piezoelectric, acoustic, phase change wax-based or thermal, have at least one printhead from which droplets of ink are directed towards a recording sheet. Within the printhead, the ink is contained in a plurality of channels. For a drop-on-demand printhead power pulses cause the droplets of ink to be expelled as required from orifices or nozzles at the end of the channels.
In a thermal ink-jet printer, the power pulses are usually produced by formation and growth of vapor bubbles on heating elements or resistors, each located in a respective one of the channels, which are individually addressable to heat and vaporize ink in the channels. As voltage is applied across a selected resistor, a vapor bubble grows in the associated channel and initially expels the ink therein from the channel orifice, thereby forming a droplet moving in a direction away from the channel orifice and towards the recording medium where, upon hitting the recording medium, a dot or spot of ink is deposited. Following collapse of the vapor bubble the channel is refilled by capillary action, which, in turn, draws ink from a supply container of liquid ink. Operation of a thermal ink-jet printer is described in, for example, U.S. Pat. No. 4,849,774.
The ink jet printhead may be incorporated into either a carriage type printer, a partial width array type printer, or a page-width type printer. The carriage type printer typically has a relatively small printhead containing the ink channels and nozzles. The printhead can be sealingly attached to a disposable ink supply cartridge and the combined printhead and cartridge assembly is attached to a carriage which is reciprocated to print one swath of information (equal to the length of a column of nozzles), at a time, on a supported, stationary recording medium, such as paper or a transparency.
After the swath is printed, the paper is stepped a distance equal to the height of the printed swath or a portion thereof, so that the next printed swath is contiguous or overlapping therewith. This procedure is repeated until an entire page is printed. In contrast, the page width printer includes a stationary printhead having a length sufficient to print across the width or length of a supported sheet of recording medium at a time. The supported recording medium is continually moved past the page width printhead in a direction substantially normal to the printhead length and at a constant or varying speed during the printing process.
In either case, the substrate or sheet is supported and heated on a heating and supporting assembly that includes a platen and a heating device in order to dry the printed swath and prevent it from bleeding into an adjacent swath. Typically, the sheet supporting platen consists of a flat surface, or of a rotating hollow drum, that in either case, has a back surface, and a front surface that has an area which is large enough to support up to a legal size sheet, with border areas left over. In the case of a rotating hollow drum platen for example, heat is generated by a radiant heater or heating device mounted inside the hollow of the drum. In order to obviate the need for costly slip rings or other like contacts, the heating device is mounted to be stationary, while the drum rotates.
The heat ordinarily is delivered to the back or inner surface of the drum uniformly, and conventionally is absorbed uniformly through the inner surface and into the wall of the drum. Conventionally too, the heat is then ordinarily emitted uniformly from the front or outer surface of the drum. Unfortunately however, heat removal from the front surface by substrates or sheets being supported on an area of the front surface, depends significantly on the particular size of the sheet, and upon the frequency at which that particular size of sheet is being used or run through the printer.
For example, by far the most frequently used paper or sheet size in North America is the letter size or 8.5".times.11" sheet. Typically, it is this sheet size that is used to base the main throughput rate specification, for example, 25 CPM (copy sheets per minute) of the ink jet printer. This letter size or 8.5".times.11" sheet unfortunately however is supported on only about 69% of the front surface area of a 9".times.15" drum or platen, that is large enough to also support, for example, 8.5".times.14", legal size sheets. Therefore, for all the time the most-run or most-used, letter size or 8.5".times.11" sheet is being run, heat is usefully taken out only from about 69% of the front surface area, while the remaining about 31% of the front surface area is unnecessarily and wastefully being overheated.